Fix the redirect-with-body test endpoint announcing a length it does not send

Tests/AsyncHTTPClientTests/AsyncAwaitEndToEndTests.swift

@@ -733,6 +733,29 @@ final class AsyncAwaitEndToEndTests: XCTestCase {
         _ = try await httpClient.execute(request, deadline: .now() + .seconds(2))
     }
 
+    func testFollowingRedirectsWithBodiesThatAreNotReadDoesNotFailTheNextRequest() {
+        XCTAsyncTest {
+            let bin = HTTPBin(.http1_1(compress: false))
+            defer { XCTAssertNoThrow(try bin.shutdown()) }
+            var config = HTTPClient.Configuration()
+            config.redirectConfiguration = .follow(max: 5, allowCycles: false)
+            let client = HTTPClient(eventLoopGroupProvider: .singleton, configuration: config)
+            defer { XCTAssertNoThrow(try client.syncShutdown()) }
+
+            // The body of a redirect is dropped without being read. This is repeated because what could go wrong is
+            // up to timing: the next request must not get a connection that is not usable.
+            for _ in 0..<150 {
+                let response = try await client.execute(
+                    HTTPClientRequest(url: "http://localhost:\(bin.port)/redirect/302-with-body?size=4096"),
+                    deadline: .now() + .seconds(10)
+                )
+                XCTAssertEqual(response.status, .ok)
+                XCTAssertEqual(response.history.count, 2)
+                _ = try await response.body.collect(upTo: 1024 * 1024)
+            }
+        }
+    }
+
     func testRedirectChangesHostHeader() {
         XCTAsyncTest {
             let bin = HTTPBin(.http2(compress: false))

Tests/AsyncHTTPClientTests/HTTPClientTestUtils.swift

@@ -807,6 +807,9 @@ internal struct HTTPResponseBuilder {
     var requestBodyByteCount: Int
     let responseBodyIsRequestBodyByteCount: Bool
     let trailers: HTTPHeaders?
+    /// Sends `body` as it is, instead of wrapping it into the JSON description of the request that other endpoints
+    /// answer with. Needed to answer with a body that has exactly the length that the response head announces.
+    var sendsBodyVerbatim = false
 
     init(
         _ version: HTTPVersion = HTTPVersion(major: 1, minor: 1),
@@ -1064,6 +1067,10 @@ internal final class HTTPBinHandler: ChannelInboundHandler {
                 headers.replaceOrAdd(name: "content-length", value: "\(size)")
                 var builder = HTTPResponseBuilder(status: .found, headers: headers)
                 builder.body = ByteBuffer(repeating: UInt8(ascii: "x"), count: size)
+                // The head announces `size` bytes, the body must have exactly that many. Wrapped into the JSON description of
+                // the request it would be longer, and the bytes after the announced length would be taken for the start of
+                // the next response.
+                builder.sendsBodyVerbatim = true
                 self.resps.append(builder)
                 return
             case "/percent%20encoded":
@@ -1160,7 +1167,9 @@ internal final class HTTPBinHandler: ChannelInboundHandler {
             var response = self.resps.removeFirst()
             response.head.headers.add(contentsOf: self.responseHeaders)
             context.write(wrapOutboundOut(.head(response.head)), promise: nil)
-            if let body = response.body {
+            if response.sendsBodyVerbatim, let body = response.body {
+                context.write(wrapOutboundOut(.body(.byteBuffer(body))), promise: nil)
+            } else if let body = response.body {
                 let requestInfo = RequestInfo(
                     data: String(buffer: body),
                     requestNumber: self.requestId,